1. Field of the Invention
The invention relates to electronic measuring and testing. More particularly, the invention relates to automatically sensing digital signals in a tested device and recording representations thereof.
2. Description of the Prior Art
Digital logic circuits, such as microprocessors, operate at high frequencies and internally generate very narrow and closely spaced pulses. Troubleshooting these circuits requires monitoring large numbers of test points for long periods of time. For example, a 4 mHz microprocessor generates pulses a few nano-seconds wide and occasionally less than a nano-second apart. Comprehensive troubleshooting requires knowledge of signal conditions at, perhaps, 100 points for many hours.
Oscillograph paper-chart recorders trace, on a moving strip, inked lines representing electrical signals present at inputs. Mechanical design limits paper chart recorders to tracing low frequency phenomena although moderate numbers of signals may be permanently recorded over extremely long time periods. Cathode-ray Oscilloscopes transiently display small numbers of signals occurring during an extremely short display window. However, the signals may vary at high frequencies. Logic analyzers transiently display more signals than Cathode-ray Oscilloscopes, and define the time window by prespecified conditions.
Digital computers monitor relatively high frequency phenomena at a large number of points and print on paper characters representing the results of calculations made on the signal values.
While the prior art teaches monitoring of large numbers of high frequency signal test points for long periods and recording signal representations, or calculations based thereon, a troubleshooter must still reduce the resulting mass of data to a form which can be analyzed. In radio communication, incoming signals are classified by modulation type in accordance with the number of consecutive identical digital values derived by instantaneously comparing signal slices with a threshold. In U.S. Pat. No. 4,166,980 (Sanders Associates, Inc.), a display shows the distribution of consecutive 0's and 1's, frequencies and the amplitudes on a channel. A zero-crossing detector, such as the one in IBM TECHNICAL DISCLOSURE BULLETIN, June 1975, page 144, can step a counter to record the number of transitions of an input signal. U.S. Pat. No. 4,079,327 (Telecommunications Radioelectriques at Telephoniques T.R.T.) discloses comparing the count with another value to indicate equality. However, the prior art does not suggest any apparatus or method for visually recording the results of monitoring a large number of high-speed signals over a long period to facilitate rapid analysis by a troubleshooter.